The invention relates to a rotary valve for power steering apparatus, and in particular, to a valve sleeve which forms part thereof.
A power steering apparatus generally comprises an input shaft which is turned by a steerable wheel, and an output shaft disposed in axial alignment with the input shaft, which are connected together by a torsion bar disposed between them and in axial alignment therewith. A rotary valve comprises a valve rotor which is formed around the outer surface of the input shaft, and a valve sleeve disposed in surrounding relationship with the valve rotor for integral rotation with the output shaft. The rotary valve is switched as a result of a relative rotation between the both shafts, thereby controlling the delivery to or discharge from a power cylinder of hydraulic fluid.
The valve sleeve which forms the rotary valve normally comprises a pipe section of an increased internal diameter and another pipe section of a reduced internal diameter. The output shaft is fitted into the pipe section of an increased diameter, with its end face disposed in opposing relationship with a step defined between the both pipe sections. A radially extending pin which is a press fit into the pipe section of an increased diameter serves connecting the valve sleeve and the output shaft together. A radially extending return oil path is formed in the valve sleeve at the step between the both pipe sections and also in the input shaft at a location situated radially inward of the step, thereby providing a communication between a flow path for the return oil which passes through the axial portion of the input shaft and over the outer surface of the valve sleeve.
In the described rotary valve, the return oil path in the valve sleeve is formed in the region of the abutment of the valve sleeve and the output shaft, or extending across the step formed in the inner surface. This causes burrs to be produced during a boring operation, and an additional machining operation is required to remove such burrs, disadvantageously causing an increased cost. In addition, use of the region of abutment between the valve sleeve and the output shaft as a return oil path causes part of the return oil path which is formed in the valve sleeve to be closed by the output shaft, resulting in disadvantages that an available channel area is reduced to give rise to the occurrence of acoustical oscillations in the fluid and that an increased pressure loss results.